Displays With Silicon and Semiconducting Oxide Thin-Film Transistors

ABSTRACT

An electronic device may include a display having an array of display pixels on a substrate. The display pixels may be organic light-emitting diode display pixels or display pixels in a liquid crystal display. In an organic light-emitting diode display, hybrid thin-film transistor structures may be formed that include semiconducting oxide thin-film transistors, silicon thin-film transistors, and capacitor structures. The capacitor structures may overlap the semiconducting oxide thin-film transistors. Organic light-emitting diode display pixels may have combinations of oxide and silicon transistors. In a liquid crystal display, display driver circuitry may include silicon thin-film transistor circuitry and display pixels may be based on oxide thin-film transistors. A single layer or two different layers of gate metal may be used in forming silicon transistor gates and oxide transistor gates. A silicon transistor may have a gate that overlaps a floating gate structure.

This application is continuation of U.S. patent application Ser. No.15,727,475, filed Oct. 6, 2017, which is hereby incorporated byreference herein in its entirety and which is a continuation of U.S.patent application Ser. No. 14/249,716, filed Apr. 10, 2014, now U.S.Pat. No. 9,818,765, which is hereby incorporated by reference herein inits entirety and which is a continuation-in-part of U.S. patentapplication Ser. No. 14/228,070, filed Mar. 27, 2014, now U.S. Pat. No.9,564,478, which is hereby incorporated by reference herein in itsentirety and which claims the benefit of U.S. provisional patentapplication Ser. No. 61/869,937, filed Aug. 26, 2013, which is herebyincorporated by reference herein in its entirety.

BACKGROUND

This relates generally to electronic devices and, more particularly, toelectronic devices with displays that have thin-film transistors.

Electronic devices often include displays. For example, cellulartelephones and portable computers include displays for presentinginformation to users.

Displays such as liquid crystal displays are formed from multiplelayers. A liquid crystal display may, for example, have upper and lowerpolarizer layers, a color filter layer that contains an array of colorfilter elements, a thin-film transistor layer that includes thin-filmtransistors and display pixel electrodes, and a layer of liquid crystalmaterial interposed between the color filter layer and the thin-filmtransistor layer. Each display pixel typically includes a thin-filmtransistor for controlling application of a signal to display pixelelectrode structures in the display pixel.

Displays such as organic light-emitting diode displays have an array ofdisplay pixels based on light-emitting diodes. In this type of display,each display pixel includes a light-emitting diode and thin-filmtransistors for controlling application of a signal to thelight-emitting diode.

Thin-film display driver circuitry is often included in displays. Forexample, gate driver circuitry and demultiplexer circuitry on a displaymay be formed from thin-film transistors.

If care is not taken, thin-film transistor circuitry in the displaypixels and display driver circuitry of a display may exhibitnon-uniformity, excessive leakage currents, insufficient drivestrengths, poor area efficiency, hysteresis, and other issues. It wouldtherefore be desirable to be able to provide improved electronic devicedisplays.

SUMMARY

An electronic device may be provided with a display. The display mayhave an array of display pixels on a substrate. The display pixels maybe organic light-emitting diode display pixels or display pixels in aliquid crystal display.

In an organic light-emitting diode display, hybrid thin-film transistorstructures may be formed that include semiconducting oxide thin-filmtransistors, silicon thin-film transistors, and capacitor structures.The capacitor structures may overlap the semiconducting oxide thin-filmtransistors. Capacitor structures may also be formed from multipleoverlapping electrode layers formed from source-drain metal layers, apolysilicon layer, and a gate metal layer may be used.

Organic light-emitting diode display pixels may have combinations ofoxide and silicon transistors. Transistors such as drive transistorsthat are coupled to light-emitting diodes may be formed from oxidetransistor structures and switching transistors may be formed fromsilicon transistor structures.

In a liquid crystal display, display driver circuitry may includesilicon thin-film transistor circuitry and display pixels may be basedon oxide thin-film transistors. A single layer or two different layersof gate metal may be used in forming silicon transistor gates and oxidetransistor gates. A silicon transistor may have a gate that overlaps afloating gate structure. Oxide transistors may be incorporated intodisplay driver circuitry.

Display driver circuitry may be configured to expose silicon transistorcircuitry to lower voltage swings than oxide transistor circuitry in anarray of display pixels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative display such as an organiclight-emitting diode display having an array of organic light-emittingdiode display pixels or a liquid crystal display having an array ofdisplay pixels in accordance with an embodiment.

FIG. 2 is a diagram of an illustrative organic light-emitting diodedisplay pixel of the type that may be used in an organic light-emittingdiode with semiconducting oxide thin-film transistors and siliconthin-film transistors in accordance with an embodiment.

FIG. 3 is a cross-sectional side view of illustrative thin-filmtransistor structures in accordance with an embodiment.

FIG. 4 is a side view of additional illustrative thin-film transistorstructures in accordance with an embodiment.

FIG. 5 is a diagram of an illustrative organic light-emitting diodedisplay pixel of the type that may include oxide and silicon thin-filmtransistors in accordance with an embodiment.

FIGS. 6, 7, and 8 are cross-sectional side views of illustrativethin-film transistor circuitry in a liquid crystal display in accordancewith an embodiment.

FIG. 9 is a diagram of an illustrative complementarymetal-oxide-semiconductor transistor inverter of the type that may beformed from a hybrid silicon-oxide transistor structure in accordancewith an embodiment.

FIG. 10 is a cross-sectional side view of an illustrative thin-filmtransistor structure of the type that may be used to form a hybridcomplementary metal-oxide-semiconductor transistor inverter inaccordance with an embodiment.

FIG. 11 is a circuit diagram of gate driver circuitry in thin-filmdisplay driver circuitry in accordance with an embodiment.

FIG. 12 is a diagram of a level shifter of the type that may be used inthe gate driver circuitry of FIG. 11 within display driver circuitry ona display in accordance with an embodiment.

FIG. 13 is a circuit diagram of an illustrative circuit that may be usedto prevent transistors within display driver circuitry on a display fromexperiencing excessive voltages in accordance with an embodiment.

FIG. 14 is a cross-sectional side view of illustrative thin-filmtransistor circuitry in a liquid crystal display in accordance with anembodiment.

FIG. 15 is a cross-sectional side view of illustrative thin-filmtransistor circuitry that includes a top gate semiconducting oxidetransistor in a liquid crystal display in accordance with an embodiment.

FIG. 16 is a cross-sectional side view of illustrative thin-filmtransistor circuitry that includes a top gate semiconducting oxidetransistor with a light shield in a liquid crystal display in accordancewith an embodiment.

FIG. 17 is a cross-sectional side view of illustrative thin-filmtransistor circuitry that includes a top gate semiconducting oxidetransistor in a liquid crystal display in accordance with an embodiment.

FIG. 18 is a cross-sectional side view of illustrative thin-filmtransistor circuitry that includes a top gate semiconducting oxidetransistor in an organic light-emitting diode display in accordance withan embodiment.

DETAILED DESCRIPTION

A display in an electronic device may be provided with driver circuitryfor displaying images on an array of display pixels. An illustrativedisplay is shown in FIG. 1. As shown in FIG. 1, display 14 may have oneor more layers such as substrate 24. Layers such as substrate 24 may beformed from planar rectangular layers of material such as planar glasslayers. Display 14 may have an array of display pixels 22 for displayingimages for a user. The array of display pixels 22 may be formed fromrows and columns of display pixel structures on substrate 24. There maybe any suitable number of rows and columns in the array of displaypixels 22 (e.g., ten or more, one hundred or more, or one thousand ormore).

Display driver circuitry such as display driver integrated circuit 16may be coupled to conductive paths such as metal traces on substrate 24using solder or conductive adhesive. Display driver integrated circuit16 (sometimes referred to as a timing controller chip) may containcommunications circuitry for communicating with system control circuitryover path 25. Path 25 may be formed from traces on a flexible printedcircuit or other cable. The control circuitry may be located on a mainlogic board in an electronic device such as a cellular telephone,computer, set-top box, media player, portable electronic device, orother electronic equipment in which display 14 is being used. Duringoperation, the control circuitry may supply display driver integratedcircuit 16 with information on images to be displayed on display 14. Todisplay the images on display pixels 22, display driver integratedcircuit 16 may supply corresponding image data to data lines D whileissuing clock signals and other control signals to supporting thin-filmtransistor display driver circuitry such as gate driver circuitry 18 anddemultiplexing circuitry 20.

Gate driver circuitry 18 may be formed on substrate 24 (e.g., on theleft and right edges of display 14, on only a single edge of display 14,or elsewhere in display 14). Demultiplexer circuitry 20 may be used todemultiplex data signals from display driver integrated circuit 16 ontoa plurality of corresponding data lines D. With this illustrativearrangement of FIG. 1, data lines D run vertically through display 14.Each data line D is associated with a respective column of displaypixels 22. Gate lines G run horizontally through display 14. Each gateline G is associated with a respective row of display pixels 22. Gatedriver circuitry 18 may be located on the left side of display 14, onthe right side of display 14, or on both the right and left sides ofdisplay 14, as shown in FIG. 1.

Gate driver circuitry 18 may assert gate signals (sometimes referred toas scan signals) on the gate lines G in display 14. For example, gatedriver circuitry 18 may receive clock signals and other control signalsfrom display driver integrated circuit 16 and may, in response to thereceived signals, assert a gate signal on gate lines G in sequence,starting with the gate line signal G in the first row of display pixels22. As each gate line is asserted, the corresponding display pixels inthe row in which the gate line is asserted will display the display dataappearing on the data lines D.

Display driver circuitry such as demultiplexer circuitry 20 and gateline driver circuitry 18 may be formed from thin-film transistors onsubstrate 24. Thin-film transistors may also be used in formingcircuitry in display pixels 22. To enhance display performance,thin-film transistor structures in display 14 may be used that satisfydesired criteria such as leakage current, switching speed, drivestrength, uniformity, etc. The thin-film transistors in display 14 may,in general, be formed using any suitable type of thin-film transistortechnology (e.g., silicon-based, semiconducting-oxide-based, etc.).

With one suitable arrangement, which is sometimes described herein as anexample, the channel region (active region) in some thin-filmtransistors on display 14 is formed from silicon (e.g., silicon such aspolysilicon deposited using a low temperature process, sometimesreferred to as LTPS or low-temperature polysilicon) and the channelregion in other thin-film transistors on display 14 is formed from asemiconducting oxide material (e.g., amorphous indium gallium zincoxide, sometimes referred to as IGZO). If desired, other types ofsemiconductors may be used in forming the thin-film transistors such asamorphous silicon, semiconducting oxides other than IGZO, etc. In ahybrid display configuration of this type, silicon transistors (e.g.,LTPS transistors) may be used where attributes such as switching speedand good drive current are desired (e.g., for gate drivers in liquidcrystal diode displays or in portions of an organic light-emitting diodedisplay pixel where switching speed is a consideration), whereas oxidetransistors (e.g., IGZO transistors) may be used where low leakagecurrent is desired (e.g., in liquid crystal diode display pixels anddisplay driver circuitry) or where high pixel-to-pixel uniformity isdesired (e.g., in an array of organic light-emitting diode displaypixels). Other considerations may also be taken into account (e.g.,considerations related to power consumption, real estate consumption,hysteresis, etc.).

Oxide transistors such as IGZO thin-film transistors are generallyn-channel devices (i.e., NMOS transistors). Silicon transistors can befabricated using p-channel or n-channel designs (i.e., LTPS devices maybe either PMOS or NMOS). Combinations of these thin-film transistorstructures can provide optimum performance.

In an organic light-emitting diode display, each display pixel containsa respective organic light-emitting diode. A schematic diagram of anillustrative organic light-emitting diode display pixel 22-1 is shown inFIG. 2. As shown in FIG. 2, display pixel 22-1 may includelight-emitting diode 26. A positive power supply voltage ELVDD may besupplied to positive power supply terminal 34 and a ground power supplyvoltage ELVSS may be supplied to ground power supply terminal 36. Thestate of drive transistor 28 controls the amount of current flowingthrough diode 26 and therefore the amount of emitted light 40 fromdisplay pixel 22-1.

To ensure that transistor 28 is held in a desired state betweensuccessive frames of data, display pixel 22-1 may include a storagecapacitor such as storage capacitor Cst. The voltage on storagecapacitor Cst is applied to the gate of transistor 28 at node A tocontrol transistor 28. Data can be loaded into storage capacitor Cstusing one or more switching transistors such as switching transistor 30.When switching transistor 30 is off, data line D is isolated fromstorage capacitor Cst and the gate voltage on terminal A is equal to thedata value stored in storage capacitor Cst (i.e., the data value fromthe previous frame of display data being displayed on display 14). Whengate line G (sometimes referred to as a scan line) in the row associatedwith display pixel 22-1 is asserted, switching transistor 30 will beturned on and a new data signal on data line D will be loaded intostorage capacitor Cst. The new signal on capacitor Cst is applied to thegate of transistor 28 at node A, thereby adjusting the state oftransistor 28 and adjusting the corresponding amount of light 40 that isemitted by light-emitting diode 26.

Organic light-emitting diode display pixels such as pixel 22-1 of FIG. 2may use thin-film transistor structures of the type shown in FIG. 3. Inthis type of structure, two different types of semiconductor are used.As shown in FIG. 3, circuitry 72 may include display pixel structuressuch as light-emitting diode cathode terminal 42 and light-emittingdiode anode terminal 44. Organic light-emitting diode emissive material47 may be interposed between cathode 42 and anode 44. Dielectric layer46 may serve to define the layout of the display pixel and may sometimesbe referred to as a pixel definition layer. Planarization layer 50 maybe formed on top of thin-film transistor structures 52. Thin-filmtransistor structures 52 may be formed on buffer layer 54 on substrate24.

Thin-film transistor structures 52 may include silicon transistor 58.Transistor 58 may be an LTPS transistor formed using a “top gate” designand may serve as a switching transistor in an organic light-emittingdiode display pixel (see, e.g., transistor 30 in pixel 22-1 of FIG. 2).Transistor 58 may have a polysilicon channel 62 that is covered by gateinsulator layer 64 (e.g., a layer of silicon oxide). Gate 66 may beformed from patterned metal (e.g., molybdenum, as an example). Gate 66may be covered by a layer of interlayer dielectric (e.g., siliconnitride layer 68 and silicon oxide layer 70). Source-drain contacts 74and 76 may contact opposing sides of polysilicon layer 62 to form thesilicon thin-film transistor 58.

Thin-film transistor structures 52 may also include thin-film transistorand capacitor structures 60. Structures 60 may include a storagecapacitor (i.e., storage capacitor Cst of FIG. 2) and an oxide thin-filmtransistor structure. The storage capacitor may have a first terminal(sometimes referred to as a plate, electrode, or electrode layer) thatis formed from polysilicon layer 62′ (patterned as part of the samelayer as layer 62). Gate insulator layer 64′, which may be an extendedportion of gate insulator layer 64, may cover terminal 62′. Thecapacitor may have a second terminal formed from metal layer 66′. Metallayer 66′ may be patterned from the same metal layer that is used informing gate 66 of transistor 58. Dielectric layers 68 and 70 may covermetal layer 66′. The thin-film transistor in structures 60 may be a“bottom gate” oxide transistor. Layer 66′, which serves as the secondterminal of capacitor Cst (i.e., node A of FIG. 2) may also serve as thegate of the oxide transistor. The oxide transistor may serve as drivetransistor 28 of FIG. 2. The “gate insulator” of the oxide transistormay be formed from the layer of interlayer dielectric (i.e., layers 68and 70). The channel semiconductor of the oxide transistor may be formedfrom oxide layer 80 (e.g., IGZO). Oxide layer 80 may overlap polysiliconcapacitor electrode layer 62′ (i.e., the oxide transistor may overlapthe capacitor), thereby saving space. Source-drain terminals 82 and 84may be formed from metal contacting opposing ends of semiconductingoxide layer 80.

Transistors such as LTPS transistors and oxide transistors may be formedwith different layouts. For example, LTPS transistors tend to have highcarrier mobilities. As a result, LTPS transistors may have relativelylong gate lengths L and relatively short gate widths to ensureappropriately low ratios of W/L to compensate for the relatively highmobility of these transistors. This may cause LTPS transistors to berelatively inefficient for pixel layout. Oxide transistors may beconstructed with W/L ratios with smaller aspect ratios (e.g., 4/4 foroxide relative to 3/30 for LTPS). Due to these layout efficiencyconsiderations, it may be preferred to use oxide transistors as thedrive transistors in display pixels 22-1. The relatively fast switchingspeed provided by LTPS transistor may make it preferable to use LTPStransistors for switching transistors such as transistor 30 of FIG. 2.

In display pixels with more transistors (e.g., three or more, four ormore, five or more, six or more, seven or more, or eight or more), theselection of which transistors are implemented using LTPS technology andwhich transistors are implemented using oxide technology may be made soas to balance transistor performance considerations between the twotypes of transistors.

When implementing driving transistors, LTPS transistors tend to exhibitlarger size (longer channel length) than oxide transistors, tend toexhibit larger dark currents than oxide transistors, and may exhibitpoorer uniformity than oxide transistors. LTPS driving transistors mayalso exhibit more hysteresis than oxide driving transistors. As a resultof these factors, it may often be advantageous to form drivingtransistors in an organic light-emitting diode display pixel from oxidetransistors. The oxide driving transistors may exhibit low leakagecurrent and minimal hysteresis.

When implementing switching transistors, LTPS transistors may be smallerthan oxide transistors, may exhibit smaller amounts of parasiticcapacitance than oxide transistors, and may exhibit lower powerconsumption than oxide transistors. As a result of factors such asthese, it may often be advantageous to form switching transistors in anorganic light-emitting diode display pixel from LTPS transistors. TheLTPS switching transistors may exhibit high switching speed and lowparasitic capacitance.

An illustrative hybrid thin-film transistor structure that may be usedin implementing both LTPS and oxide transistors in a single organiclight-emitting diode display pixel (e.g., to implement a circuit such asdisplay pixel circuit 22-1 of FIG. 2) is shown in FIG. 4. Hybridthin-film transistor structures 114 of FIG. 4 include silicon thin-filmtransistor 108, capacitor (Cst) 110, and oxide transistor 112. Silicontransistor 108 is formed from polysilicon layer 90. Gate insulator layer92 covers polysilicon layer 90. A layer of gate metal is patterned ontop of gate insulator layer 92 to form gate 94, capacitor electrode 96,and gate electrode 98. A layer of interlayer dielectric material such assilicon nitride layer 116 and silicon oxide layer 118 may cover thepatterned gate metal structures. Source-drain contacts 100 and 94 forsilicon transistor 108 may contact (i.e., may be shorted to) polysiliconlayer 90 in the vicinity of channel region 106. Gate 94 of transistor108 may serve as an implant mask to allow low-density drain implants tobe formed in polysilicon layer 90 in regions 104 adjacent to polysiliconchannel region 106 of transistor 108.

Source-drains 100 and 102 of silicon transistor 108, capacitor electrode120, and source-drains 122 and 124 of oxide transistor 112 may be formedfrom patterned portions of a common metal layer on interlayer dielectric116 and 118.

Capacitor 110 may have a first terminal formed from metal electrode 120and from portion 126 of polysilicon layer 90. Capacitor 110 may have asecond terminal formed from metal electrode 96.

Oxide transistor 112 may have a semiconductor oxide layer such as anIGZO layer 128, source-drain contacts 122 and 124, and gate 98. Gate 98is separated from semiconductor oxide 128, which serves as the channelregion for transistor 112 by dielectric 116 and 118. Dielectric 116 and118 therefore serves as the gate insulator for oxide transistor 112.

FIG. 5 is a circuit diagram of another illustrative organiclight-emitting diode pixel circuit that may be used in display 14. Pixel22-2 includes driver transistor 28 for driving current intolight-emitting diode 26. Storage capacitor Cst is used to store signalson the gate of transistor 28 between frames. Sensing line SENSING usedto implement a compensation scheme to adjust for pixel-to-pixelvariations in transistor performance. Gate lines SCAN and SCAN2 are usedin applying control signals to switching transistors 30-1 and 30-2.

To optimize performance in display pixel 22-2, it may be desirable touse hybrid structures of the type shown in FIGS. 3 and 4 or otherconfigurations for forming silicon and/or oxide thin-film transistorsand capacitors. For example, it may be desirable to form drivetransistor 28 from an oxide transistor (e.g., an NMOS oxide transistor),while forming switching transistors such as transistors 30-1 and 30-2from silicon transistors or from a mixture of silicon (NMOS and/or PMOS)and oxide (NMOS) transistors.

With a first illustrative configuration, transistor 30-1 is an oxidetransistor, transistor 30-2 is an oxide transistor, and transistor 28 isan oxide transistor. With a second illustrative configuration,transistor 30-1 is a silicon transistor, transistor 30-2 is a silicontransistor, and transistor 28 is an oxide transistor. A hybridtransistor structure such as the structure of FIG. 3 or the structure ofFIG. 4 may be used in this scenario (e.g., to implement transistors 30-1and 28 and capacitor Cst). With an illustrative third configuration,transistor 30-1 is a silicon transistor, transistor 30-2 is an oxidetransistor, and transistor 28 is an oxide transistor. As with the secondillustrative configuration, a hybrid transistor structure such as thestructure of FIG. 3 or the structure of FIG. 4 may be used to implementtransistors 30-1 and 28 and capacitor Cst.

If desired, display 14 may be a liquid crystal display. In this type ofscenario, each pixel of display 14 may contain an electrode structurefor applying an electric field to an associated portion of a liquidcrystal layer in the display, a capacitor for storing charge on theelectrode between frames of image data, and a thin-film transistor forcontrolling the application of the electric field to the electrodes.With one suitable arrangement, gate driver circuitry 18 anddemultiplexer circuitry 20 (FIG. 1) in the liquid crystal display may beformed from silicon transistors and the thin-film transistors in displaypixels 22 may be formed from oxide transistors. The silicon transistorshave high mobility channel regions and are well suited for fastswitching speeds and high drive currents while operating at low voltagesand low power. The oxide thin-film transistors in display pixels 22exhibit low leakage currents.

Thin-film transistor structures of the type that may be used in forminga liquid crystal display with both silicon and oxide transistors areshown in FIG. 6. As shown in FIG. 6, thin-film transistor structures 242may include silicon thin-film transistor structures 216 (e.g., forforming parts of peripheral circuits such as display driver circuitry 18and demultiplexer circuitry 20) and oxide thin-film transistorstructures 240 (e.g., for forming display pixels 22 in a liquid crystaldisplay having a layout of the type shown by display 14 of FIG. 1).

Structures 216 and 240 may be formed on buffer layer 202 on substrate24. Polysilicon layer 204 may be deposited on buffer 202. Gate insulatorlayer 206 may be formed on polysilicon layer 204. A common layer ofmetal may be patterned to form metal structures 218, 220, and 228.Structure 218 may serve as the gate for a silicon transistor thatincludes source-drain contacts 212 and 214 and a channel formed frompolysilicon 204. Metal structure 228 may serve as a gate for an oxidetransistor formed from semiconducting oxide layer 224 (e.g., IGZO) andsource-drain terminals 222 and 226. Metal structure 228 may also serveas a light shield that helps block backlight in display 14 from reachingoxide layer 224, so no separate light shielding structures need beincorporated in structures 240. Interlayer dielectric such as siliconnitride layer 208 and 210 may cover gate 218 in structure 216 and mayserve as a gate insulator for gate 228 in the oxide transistor ofstructures 240.

Metal 230 contacts source-drain 226 of the display pixel thin-film oxidetransistor that is formed from oxide layer 224. Metal 230 may besupported by organic layer 232. On the surface of organic layer 232,metal 230 may form an electrode with multiple fingers. Dielectric layer236 may isolate electrode 230 from common electrode (Vcom) 234. Duringoperation, electric fields are produced between electrode 230 andelectrode 234. These fields pass through the liquid crystal material inthe display. If desired, display 14 may incorporate capacitive touchsensors that are formed from portions of Vcom electrode 234. In thistype of configuration, optional metal lines such as line 238 may be usedto help reduce the resistance of the material used in forming electrode234 (which may be, for example, a somewhat resistive conducting materialsuch as indium tin oxide).

The thickness of layers 208 and 210 may be about 6000 angstroms. Thisrelatively large thickness may help minimize capacitance between gate218 and nearby metal structures such as source-drain 214, but may limitswitching speeds in the oxide transistor. To address this concern, adesign of the type used by structures 242′ in FIG. 7 may be used. Withthe FIG. 7 arrangement, an additional semiconductor fabrication mask maybe used to create a gate for the oxide transistor that is formed from aseparate layer of metal from the metal layer used in forming gate 218.With this approach, only a single 3000 angstrom dielectric layer 210′(formed, e.g., from sublayers of silicon nitride and silicon oxide) isused to separate oxide transistor gate 228′ from oxide layer 224, sooxide transistor switching speed may be enhanced. The arrangement ofstructures 242′ of FIG. 7 allows gate 218 and gate 228′ to be formedfrom different metals. For example, gate 218 may be formed from arefractory metal such as Mo to accommodate the elevated temperaturesassociated with activating the silicon transistor, whereas gate 228′ maybe formed from a lower resistance metal such as copper.

In some applications, the handling of high drive voltages(gate-to-source and drain) may need to be considered. Transistorstructures 242″ of FIG. 8 may be used in scenarios in which it isdesired to handle relatively large (e.g., 20 volt) swings on the silicontransistor gate. In this situation, gate insulator layer 206 may beinsufficiently thin to withstand damage from a 20 volt signal. Forexample, gate insulator 206 may be about 800 angstroms thick, which maynot be sufficiently thick to reliably handle 20 volt drive voltages. Toensure that gate insulator layer 206 is not overly stressed, gatestructure 218 may be converted into a floating (electrically isolated)metal structure and an additional metal layer (i.e., part of the samemetal layer that is patterned to form gate 228′ of oxide transistor 240)may be used in forming silicon transistor gate 218′. Floating gate 218may be retained to serve as a mask for low density drain (LDD) implantsmade into the source and drain contact portions of polysilicon layer204, even though floating gate 218 is not driven with control signalsduring operation of silicon transistor 216.

In a hybrid silicon/oxide liquid crystal display, it is not necessary toform display driver circuitry such as gate driver circuitry 18 anddemultiplexer circuitry 20 from silicon transistors. If desired, some ofthis display driver circuitry may be formed from oxide transistors. Forexample, low drive current CMOS-type circuits in the peripheralcircuitry of display 14 such as illustrative CMOS inverter 300 of FIG. 9may include oxide transistors. It may be challenging to form PMOS oxidetransistors, so circuits such as inverter 300 may, if desired, be formedusing an NMOS oxide transistor and a PMOS silicon transistor (as anexample).

Hybrid oxide-silicon thin-film transistor structures such asillustrative thin-film transistor structures 302 of FIG. 10 may be usedin forming CMOS-type circuitry in display driver circuitry such as gatedriver circuitry 18 and demultiplexer circuitry 20. As shown in FIG. 10,structures 302 may have a polysilicon layer 308 that is formed onsubstrate 24. P-channel active area 310 may be formed under gate 312.Gate insulator layer 306 (e.g., silicon oxide) may separate gate 312from silicon channel region 310 in silicon layer 308. Dielectric layer302 (e.g., sublayers of silicon oxide and silicon nitride) may covergate 312. Dielectric layer 306 may separate gate 312 from overlappingoxide layer 312. Oxide layer 312 may be a semiconducting oxide such asIGZO material. Gate 312 may be formed from a first patterned metallayer. A second patterned metal layer may be used in forming outputterminal 322, source terminal 316, and drain terminal 318. Passivationlayer 320 may cover terminals 316 and 312. Gate 312 may be formed frommaterials such as molybdenum, molybdenum tungsten, tungsten, or othermetals. Metal for forming structures such as metal structures 322, 316,and 318 may be formed from metal such as aluminum, molybdenum, etc.

With the arrangement of FIG. 10, gate 314 serves as a common (shared)gate for two transistors. In particular, gate 314 (see, e.g., terminalVin of FIG. 9) serves as both a gate for a PMOS silicon transistor(transistor TP of FIG. 9) that is formed from silicon layer 308 and as agate for an NMOS oxide transistor (transistor TN of FIG. 9) that isformed from oxide layer 312. Oxide layer 312 is located above gate 314and silicon layer 310 is located below gate 314. The shared gatearrangement of FIG. 10 allows a CMOS inverter of the type shown in FIG.9 to be implemented compactly.

FIG. 11 shows illustrative gate driver circuitry 18 that may be used ona liquid crystal display. Circuitry 18 may use signals with a relativelysmall voltage swing (e.g., a 15 volt or 16 volt swing) for silicontransistors while producing gate signals G with a larger voltage swing(e.g., a 20 volt swing or more) to ensure satisfactory operation ofoxide thin-film transistors in display pixels 22 that are being drivenby the gate signals.

As shown in FIG. 11, circuitry 18 may have a shift register formed froma series of linked SR latches 400 or other register circuits. Each rowof circuitry in FIG. 11 is associated with a separate row of displaypixels 22 in a liquid crystal display and provides a respective gatesignal G for that row of display pixels. During operation, triggersignal TRIGGER may be applied to the latch in the first row of the shiftregister in circuitry 18 while a clock signal LOAD CLOCK is beingapplied to the shift register. The trigger signal causes a cascadingsignal to ripple down through the shift register. In response, eachlatch 400 asserts its output OUT in sequence. Each row of gate drivercircuitry 18 has a respective level shifter 404 and buffer 404 thatreceive output signal OUT.

Output signal OUT ranges from a high voltage of 15 V (or other suitablevoltage) to 0 volts (or other suitable voltage). The 15 volt swing thatis associated with this type of configuration can be tolerated bysilicon thin-film transistors in latches 400, whereas larger voltageswings such as 20 volt swings might overly stress the silicon thin-filmtransistors. Level shifter 402 shifts the 15 volt to 0 volt signal OUTfrom latch 400 so that the output on path 406 from level shifter 402ranges from 5 volts to −11 volts (i.e., a swing of 16 volts that can betolerated by the silicon transistors in level shifter 402). Buffer 404receives the 15 volt to 0 volt signal OUT from latch 400 as input signalIN_H and receives the 5 volt to −11 volt signal as input signal IN_L.Buffer 404 preferably contains silicon thin-film transistors. The designof buffer 404 allows buffer 404 to produce an output signal (gate linesignal G) with a large voltage swing (e.g., 15 volts to −11 volts) ofthe type that is appropriate for controlling oxide transistors in thearray of display pixels 22 on the liquid crystal display.

FIG. 12 is a circuit diagram of an illustrative circuit of the type thatmay be used to implement level shifter 402. Signals from output OUT oflatch 400 may be received at input 410 of level shifter 402 andcorresponding level-shifted output signals (signals IN_L) for buffer 404may be provided at output 412 of level shifter 402. Other level shifterdesigns may be used for level shifter 402 if desired. The configurationof FIG. 12 is merely illustrative. Silicon thin-film transistors may beused in forming level shifter 402.

Circuitry 404 of FIG. 13 is an example of a design that may be used inimplementing buffer 404 of FIG. 11. With this design, signals IN_H andIN_L are identical square wave pulses with different respective voltageswings. Signal IN_H ranges from 15 to 0 volts. Signal IN_L ranges from 5to −11 volts. Corresponding output signal (gate line signal) G in thisexample is a square wave pulse that ranges from 15 volts to −11 voltsand therefore has a swing of more than 20 volts.

Ground voltage GND is applied to the gates of transistors T2 and T3.This limits that maximum voltage experienced by the transistors ofcircuit 414 to less than about 16 volts, even though the output swing ofcircuit 414 is more than 20 volts. The ground voltage GND on the gatesof transistors T2 and T3 causes these transistors to turn off to protecttransistors T1 and T4 whenever excessive source terminal voltage swingis detected. Consider, as an example, transistors T1 and T2. TransistorT2 may be characterized by a threshold voltage Vth. If the source S oftransistor T1 starts to fall below voltage GND-Vth, transistor T2 willturn off and isolate transistor T1. Transistors T3 and T4 operate in thesame way. Using this arrangement, none of the transistors in buffer 414is exposed to excessive voltage swings, allowing transistors T1, T2, T3,and T4 to be formed from silicon thin-film transistors.

If desired, other circuit configurations may be used to allow gatedriver circuitry 18 to operate in an environment in which gate linesignal G has a large voltage swing to accommodate oxide transistors indisplay pixels 22. As an example, a subset of the level shiftertransistors and a subset of the output buffer transistors may beimplemented using oxide thin-film transistor structures in addition tousing silicon thin-film transistor structures.

FIG. 14 is a cross-sectional side view of additional thin-filmtransistor circuitry of the type that may be used in a liquid crystaldisplay. As shown in FIG. 14, thin-film transistor structures 242 mayinclude silicon thin-film transistor structures 216 (e.g., for formingparts of peripheral circuits such as display driver circuitry 18 anddemultiplexer circuitry 20) and oxide thin-film transistor structures240 (e.g., for forming display pixels 22 in a liquid crystal displayhaving a layout of the type shown by display 14 of FIG. 1).

Structures 216 and 240 may be formed on buffer layer 202 on substrate24. Polysilicon layer 204 may be deposited on buffer 202. Gate insulatorlayer 206 may be formed on polysilicon layer 204. A common layer ofmetal may be patterned to form metal structures 218, 220, and 228.Structure 218 may serve as the gate for a silicon transistor thatincludes source-drain contacts 212 and 214 and a channel formed frompolysilicon 204. Metal structure 228 may serve as a gate for an oxidetransistor formed from semiconducting oxide layer 224 (e.g., IGZO) andsource-drain terminals 222 and 226. Metal structure 228 may also serveas a light shield that helps block backlight in display 14 from reachingoxide layer 224, so no separate light shielding structures need beincorporated in structures 240. Interlayer dielectric such as siliconnitride layer 208 and 210 may cover gate 218 in structure 216 and mayserve as a gate insulator for gate 228 in the oxide transistor ofstructures 240.

Metal structures 218, 220, and 228 and routing lines such asinterconnect line 502 may be formed from a first metal layer (sometimesreferred to as an M1 layer). Metal 222 and 226, which form source-draincontacts for the oxide transistor of structures 240, and routing linessuch as interconnect line 500 may be formed from a second metal layer(sometimes referred to as an SD1 layer). Metal structures 212, 214, androuting lines such as interconnect line 506 may be formed from a thirdmetal layer (sometimes referred to as an SD2 layer). Dielectric layers232B may separate the second metal layer from the third metal layer.Dielectric layer 232A may separate the third metal layer from metalstructures such as metal layer 234.

Metal 230 contacts metal layer 504 and is thereby coupled tosource-drain 226 of the display pixel thin-film oxide transistor that isformed from oxide layer 224. Metal 230 may be supported by organic layer232B. On the surface of organic layer 232B, metal 230 may form anelectrode with multiple fingers. Dielectric layer 236 may isolateelectrode 230 from common electrode (Vcom) 234. During operation,electric fields are produced between electrode 230 and electrode 234.These fields pass through the liquid crystal material in the display. Ifdesired, display 14 may incorporate capacitive touch sensors that areformed from portions of Vcom electrode 234. In this type ofconfiguration, optional metal lines such as line 238 may be used to helpreduce the resistance of the material used in forming electrode 234(which may be, for example, a somewhat resistive conducting materialsuch as indium tin oxide).

Capacitive coupling between the routing lines in display 14 can lead toswitching losses. As an example, source-drain structure 222 may becoupled to the data line in display 14. The voltage on this lineswitches relative to Vcom (electrode 234) and can lead to power losses.The presence of dielectric layers 232A and 232B can help reducecapacitive coupling between the data line and Vcom electrode and therebyreduce power losses. The presence of these dielectric layers can alsoreduce capacitive coupling between routing lines in display 14 (e.g.,capacitive coupling between routing lines and other structures of thefirst and second metal layers, the first and third metal layers, etc.).Layers 232A and 232B may be formed from low-dielectric-constant organicdielectric or other dielectric material. As an example, layers 232A and232B may be acrylic polymers, other polymers, dielectrics of the typesometimes referred to as spin-on-glass, (e.g., spin-on-glass polymersdeposited via slit coating tools, etc.), siloxane-based materials, etc.

FIG. 15 is a cross-sectional side view of illustrative thin-filmtransistor circuitry for a liquid crystal display that includes a topgate semiconducting oxide transistor. As shown in FIG. 15, thin-filmtransistor structures 242 may include silicon thin-film transistorstructures 216 and semiconducting oxide thin-film transistor structures240. Silicon thin-film transistor structures 216 may be used inperipheral circuits such as display driver circuitry 18 anddemultiplexer circuitry 20 and/or may be used in forming circuits fordisplay pixels 22 in a liquid crystal display. Semiconducting oxidethin-film transistor structures 240 may be used in peripheral circuitssuch as display driver circuitry 18 and demultiplexer circuitry 20and/or may be used in forming circuits for display pixels 22 in a liquidcrystal display. Transistors such as silicon (polysilicon) transistor216 may be n-channel or p-channel devices. Transistors such assemiconducting oxide transistor 240 may be n-channel or p-channeldevices.

Structures 216 and 240 may be formed on buffer layer 202 on substrate24. Buffer layer 202 may be formed from a dielectric such as aninorganic dielectric. Buffer layer 202 may help prevent ions insubstrate 24 from migrating into structures 216 and 240.

Polysilicon layer 204 may be deposited on buffer 202. Gate insulatorlayer 206 may be formed on polysilicon layer 204. Gate insulator layer206 may be formed from a dielectric such as silicon oxide (e.g., a 100nm silicon oxide layer). A common layer of metal may be patterned toform metal structures 218, 220, and 228. Structure 218 may serve as thegate for a silicon transistor that includes source-drain contacts 212and 214 and a channel formed from polysilicon 204. Metal structure 228may serve as a gate for a top gate oxide transistor (i.e., asemiconducting oxide transistor) formed from semiconducting oxide layer224 (e.g., IGZO) and source-drain terminals 222 and 226. One or morelayers of interlayer dielectric (ILD) may cover metal structures 218,220, and 228. For example, a first dielectric layer such as layer 208and a second dielectric layer such as layer 210 may cover metalstructures 218, 220, and 228. Layer 208 may be a silicon nitride layerand layer 210 may be a silicon oxide layer (as examples). Because thereis no lateral overlap between gate 228 and source-drain electrodes 222and 226, parasitic capacitance between gate 228 and source-drainstructures 222 and 226 may be minimized. Moreover, layers 208 and 210 ofthe oxide transistor of FIG. 15 may be thicker than layers 208 and 210in bottom-gate oxide transistor of FIG. 14, thereby further reducingparasitic capacitances.

Metal structures 218, 220, and 228 may be formed from a first metallayer (sometimes referred to as an M1 layer). Metal 222 and 226, whichform source-drain contacts for the oxide transistor of structures 240and metal 212 and 214, which form source-drain contacts for the silicontransistor of structures 216 may be formed from a second metal layer(sometimes referred to as an SD1 layer or M2 layer). Metal structuressuch as metal line 238 may be formed from a third metal layer (sometimesreferred to as an M3 layer). Dielectric 232 (e.g., an organic dielectriclayer such as a polymer layer) may separate the second metal layer fromthe third metal layer.

Metal 230 contacts source-drain 226 of the display pixel thin-film oxidetransistor that is formed from oxide layer 224. Metal 230 may besupported by organic layer 232. On the surface of organic layer 232,metal 230 may form an electrode with multiple fingers (e.g., a pixelelectrode for a display pixel in the display). Dielectric layer 236 mayisolate electrode 230 from common electrode (Vcom) 234. Duringoperation, electric fields are produced between electrode 230 andelectrode 234. These fields pass through the liquid crystal material inthe display that is formed on top of the structures of FIG. 15. Ifdesired, display 14 may incorporate capacitive touch sensors that areformed from portions of Vcom electrode 234. In this type ofconfiguration, optional metal lines such as line 238 may be used to helpreduce the resistance of the material used in forming electrode 234(which may be, for example, a somewhat resistive conducting materialsuch as indium tin oxide).

As shown in FIG. 16, an optional light shielding structure such as lightshield 520 can be formed under semiconducting-oxide transistor 240 orelsewhere in the display. Light shield 520 may be formed from an opaquematerial such as a metal, an oxidized metal, a dark polymer, or otherlight blocking materials. The presence of light shield 520 can helpprevent stray light from disrupting the operation of semiconductingoxide transistor structures 240 or other overlapping structures.

In the example of FIG. 17, dielectric layer 232 of FIG. 15 has beendivided into two dielectric layers 232A and 232B. Layer 232A may overlapthe source-drain electrodes of transistors 216 and 240. Layer 232B maybe interposed between the source-drain electrodes and other metalstructures formed form the source-drain metal layer and layers 208 and210. As described in connection with FIG. 14, this type of two-layerapproach can reduce capacitive coupling between the metal structures ofdevices 216 and 240. A cross-sectional side view of illustrativethin-film transistor circuitry that includes a top gate semiconductingoxide transistor in an organic light-emitting diode display is shown inFIG. 18. As shown in FIG. 18, circuitry 72 may include display pixelstructures such as light-emitting diode cathode terminal 42 andlight-emitting diode anode terminal 44. Organic light-emitting diodeemissive material 47 may be interposed between cathode 42 and anode 44.Pixel definition layer 46 may be a dielectric layer 46 that serves todefine the layout of the display pixel. Layer 46 may be formed from apolymer such as a black polymer to help block stray light.

Planarization layer 50 may be formed on top of thin-film transistorstructures 52. Thin-film transistor structures 52 may be formed onbuffer layer 54 on substrate 24. Substrate 24 may be formed from metal,glass, polymer, other materials, or combinations of these materials.Buffer layer 54 may be formed from an inorganic dielectric layer thathelps prevent ions in substrate 24 from disrupting the operation ofstructures 52. Optional functional layer 522 may be interposed betweenbuffer layer 54 and substrate 24. Functional layer 522 may be a stressrelief layer, a light-blocking layer, a layer used in forming componentssuch as capacitors (e.g., capacitor electrodes for pixel circuits and/orperipheral circuits), etc.

Thin-film transistor structures 52 may include silicon transistor 58.Transistor 58 may be an LTPS transistor formed using a top gate designand may serve as a switching transistor in an organic light-emittingdiode display pixel (see, e.g., transistor 30 in pixel 22-1 of FIG. 2).Transistor 58 may also be used in peripheral circuits (e.g., drivercircuitry 18 and demultiplexer circuitry 20).

Transistor 58 may have a polysilicon channel 62 that is covered by gateinsulator layer 64 (e.g., a layer of silicon oxide having a thickness of100 nm or other suitable thickness). Gate 66 may be formed frompatterned metal (e.g., molybdenum, as an example). Gate 66 may becovered by a layer of interlayer dielectric (e.g., silicon nitride layer68 and silicon oxide layer 70). Source-drain contacts 74 and 76 maycontact opposing sides of polysilicon layer 62 to form silicon thin-filmtransistor 58.

Dielectric layer 526 may cover source-drain structures 74 and 76.Optional metal layer 524 may be formed on layer 526 and may, if desired,contact underlying metal structures though vias (see, e.g., vias 528).Structure 66 may be formed in a first (“M1”) metal layer. Source-drainelectrodes 74 and 76 may be formed in a second metal layer. Metal layer524 may be formed as part of a third (“M3”) metal layer. Layer 524 mayoverlap portions of transistor 58 and/or transistor 60 and may be usedfor forming capacitors or signal interconnect lines (i.e., routing).Layer 524 may be overlapped by emissive material layer 47 and may form alight-blocking structures that prevent stray light from emissivematerial 47 from reaching underlying transistor structures, etc.

Thin-film transistor structures such as semiconducting-oxide thin-filmtransistor structures 60 and silicon thin-film transistor structures 58may be used in forming part of a pixel circuit in an organiclight-emitting diode display and/or may be used in forming part ofperipheral circuitry 18 and 20. Thin-film transistor 60 of FIG. 18 maybe a top gate semiconducting-oxide transistor. The gate insulator layer64, which serves as the gate insulator for silicon transistor 58, alsoserves as the gate insulator for oxide transistor 60.

Metal gate 532 forms the gate of oxide transistor 60. The channelsemiconductor of the oxide transistor may be formed from semiconductingoxide layer 128 (e.g., IGZO). Source-drain terminals 534 and 536 may beformed from metal contacting opposing ends of semiconducting oxide layer128. Metal structures 530 and 538 may be used for routing and may beformed from the same layer of metal that is pattered to form gates 66and 532. Structures such as source-drain structures 534 and 536 may beformed from the same layer of metal that is used in forming source-drainstructures 74 and 76.

The foregoing is merely illustrative and various modifications can bemade by those skilled in the art without departing from the scope andspirit of the described embodiments. The foregoing embodiments may beimplemented individually or in any combination.

What is claimed is:
 1. A display pixel comprising: a first power supplyterminal; a second power supply terminal; a light-emitting diode coupledbetween the first and second power supply terminals; a drive transistorcoupled between the first power supply terminal and the light-emittingdiode, wherein the drive transistor is an oxide transistor; and aswitching transistor that is coupled to a node that is interposedbetween the drive transistor and the light-emitting diode, wherein theswitching transistor is an oxide transistor.
 2. The display pixeldefined in claim 1, wherein the switching transistor is a firstswitching transistor and wherein the display pixel further comprises: asecond switching transistor that is coupled to the gate of the drivetransistor, wherein the second switching transistor is an oxidetransistor; and a capacitor with first and second terminals, wherein thefirst terminal is coupled between the gate of the drive transistor andthe second switching transistor and the second terminal is coupledbetween the drive transistor and the light-emitting diode.
 3. Thedisplay pixel defined in claim 1, further comprising: a capacitor withfirst and second terminals, wherein the first terminal is coupled to agate of the drive transistor and the second terminal is coupled betweenthe drive transistor and the light-emitting diode.
 4. The display pixeldefined in claim 1, wherein the switching transistor is a firstswitching transistor and wherein the display pixel further comprises asecond switching transistor that is coupled to the gate of the drivetransistor.
 5. The display pixel defined in claim 4, further comprising:a capacitor with first and second terminals, wherein the first terminalis coupled between the gate of the drive transistor and the secondswitching transistor and the second terminal is coupled between thedrive transistor and the light-emitting diode.
 6. The display pixeldefined in claim 4, wherein the second switching transistor is an oxidetransistor.
 7. The display pixel defined in claim 1, wherein theswitching transistor is an n-type metal-oxide-semiconductor transistorthat has a channel formed from a layer of semiconducting oxide.
 8. Thedisplay pixel defined in claim 7, wherein the drive transistor is ann-type metal-oxide-semiconductor transistor that has a channel formedfrom a layer of semiconducting oxide.
 9. A display pixel comprising: afirst power supply terminal; a second power supply terminal; alight-emitting diode coupled between the first and second power supplyterminals; a drive transistor coupled between the first power supplyterminal and the light-emitting diode, wherein the drive transistor isan oxide transistor; and a switching transistor that is coupled to anode that is interposed between the drive transistor and thelight-emitting diode, wherein the switching transistor is a silicontransistor.
 10. The display pixel defined in claim 9, wherein theswitching transistor is a first switching transistor and wherein thedisplay pixel further comprises: a second switching transistor that iscoupled to the gate of the drive transistor, wherein the secondswitching transistor is an oxide transistor; and a capacitor with firstand second terminals, wherein the first terminal is coupled between thegate of the drive transistor and the second switching transistor and thesecond terminal is coupled between the drive transistor and thelight-emitting diode.
 11. The display pixel defined in claim 9, furthercomprising: a capacitor with first and second terminals, wherein thefirst terminal is coupled to a gate of the drive transistor and thesecond terminal is coupled between the drive transistor and thelight-emitting diode.
 12. The display pixel defined in claim 9, whereinthe switching transistor is a first switching transistor and wherein thedisplay pixel further comprises a second switching transistor that iscoupled to the gate of the drive transistor.
 13. The display pixeldefined in claim 12, further comprising: a capacitor with first andsecond terminals, wherein the first terminal is coupled between the gateof the drive transistor and the second switching transistor and thesecond terminal is coupled between the drive transistor and thelight-emitting diode.
 14. The display pixel defined in claim 12, whereinthe second switching transistor is an oxide transistor.
 15. The displaypixel defined in claim 14, wherein the switching transistor is a p-typemetal-oxide-semiconductor transistor that has a channel formed from alayer of polysilicon.
 16. The display pixel defined in claim 9, whereinthe drive transistor is an n-type metal-oxide-semiconductor transistorthat has a channel formed from a layer of semiconducting oxide.
 17. Thedisplay pixel defined in claim 9, wherein the switching transistor is ap-type metal-oxide-semiconductor transistor that has a channel formedfrom a layer of polysilicon.
 18. A display pixel comprising: a firstpower supply terminal; a second power supply terminal; a light-emittingdiode coupled between the first and second power supply terminals; adrive transistor coupled between the first power supply terminal and thelight-emitting diode, wherein the drive transistor is an oxidetransistor; a first switching transistor that is coupled to a gate ofthe drive transistor, wherein the first switching transistor is an oxidetransistor; a capacitor having a first terminal coupled between thefirst switching transistor and the drive transistor and a secondterminal coupled between the drive transistor and the light-emittingdiode; and a second switching transistor that is coupled between thedrive transistor and the light-emitting diode.
 19. The display pixeldefined in claim 18, wherein the second switching transistor is a p-typemetal-oxide-semiconductor transistor that has a channel formed from alayer of polysilicon.
 20. The display pixel defined in claim 18, whereinthe second switching transistor is an n-type metal-oxide-semiconductortransistor that has a channel formed from a layer of semiconductingoxide.